prosencephalon

The prosencephalon, or forebrain, is the anterior primary vesicle of the neural tube in early brain development. It serves as the embryonic precursor to the forebrain structures of the mature brain.

During the third to fourth week of gestation, the prosencephalon divides into the telencephalon and diencephalon. The telencephalon gives rise to the cerebral hemispheres, including the cerebral cortex and basal ganglia, while the diencephalon forms the thalamus, hypothalamus, and related structures; it also contributes to the optic apparatus via the optic vesicles.

In the developing brain, the remaining primary vesicles are the mesencephalon (midbrain) and rhombencephalon (hindbrain). As development proceeds, the telencephalon expands to enclose the lateral ventricles, and the diencephalon contains the third ventricle.

Malformations of the prosencephalon can result in holoprosencephaly, a spectrum of forebrain defects characterized by inadequate division of the forebrain into two hemispheres. Severity ranges from alobar to semilobar and lobar forms and may be associated with craniofacial anomalies.

Etymology and usage: The term derives from Greek pro-"before" and enkephalon meaning brain. In embryology, prosencephalon is used to denote the forebrain portion of the neural tube that gives rise to the telencephalon and diencephalon.